1. Field of the Invention
The present invention relates generally to a mask support assembly of a color CRT(Cathode Ray Tube), and more particularly to a tension aperture grill mask which is placed at the inside surface of a screen of a flat CRT and serves to discriminate colors and a tension mask assembly of the flat CRT which supports the aperture grill mask in the state where a predetermined tension is maintained.
2. Description of the Related Art
Generally, the front panel for picture display in a flat CRT is formed to be flat, the electron beam emitted from an electron gun strikes a phosphor screen formed at the inside of a screen in a predetermined pattern with executing a uniform motion, and thus the phosphor is radiated so as to embody the pictures.
A tension aperture grill mask of thin film (hereinafter, referred to as a tension mask) which maintains a predetermined distance from the phosphor screen is combined at the inner surface of the panel of the flat CRT in the state where a predetermined tension is maintained by an additional support assembly, so as to discriminate the colors of the emitted electron beam.
The flat CRT of such construction prevents the image distortion which occurs depending on the angle at which a viewer watches a television by making the curvature of the screen be flat as compared with a general CRT, and it is possible to maximize the effective visual angle to the visual angle of 180xc2x0 right and left.
It also has an advantage that it can prevent the external light from being reflected to the eyes of a viewer by minimizing the reflection of the screen due to the external light and thus diminish the eye strain even if the viewer watches the television for a long time.
A description will be made on a flat CRT to which a general tension mask is applied with reference to FIGS. 1 to 3.
The outer appearance of the flat CRT is formed by a panel 2 at an inner surface of which a phosphor screen 1 is formed and a funnel 3 which is joined at the rear of the panel 2 by a frit glass.
An electron gun 5 for emitting three electron beams 6 of R, G and B is disposed in a neck portion 4 of the funnel 3, and a deflection yoke 7 is placed at one side of the neck portion 4 so as to radiate the electron beams 6 emitted from the electron gun 5 to the screen.
A tension mask 7 having an aperture grill 11 for discriminating the colors of the emitted electron beams is placed at the inner surface of the panel 2, and the tension mask 7 is supported by a support assembly 8 which is disposed with maintaining a predetermined distance from the inner surface of the panel 2. The support assembly 8 is fixed against a stud pin 9 formed at the inner surface of the panel 2 by a support spring 10.
As shown in FIG. 2, the tension mask 7 is made of a thin metal plate (about 0.1xcx9c0.2 mm), and has a vertical (or horizontal ) slot-shaped aperture grill 11 through which the electron beams pass. The edges arranged at both sides of the aperture grill 11 are stretched by the tension applied from the support assembly 8 and are welded to both rails 12 of the support assembly 8.
The support assembly 8 is composed of a rail 12 which has two long sides in a horizontal direction and an L-shaped section, and a support bar 13 which has two short sides in a vertical direction and is welded to both ends of the rail 12 so as to support the rail 12.
The support bar 13 serves to expand the rail 12 to the outside direction against the vertical direction together with the function of supporting the rail 12, and also serves to stretch and tension the tension mask 7 welded at the rail 12 when the rail 12 is expanded to the outside direction.
A plurality of damper wires 14 which cross the effective surface of the tension mask 7 in a direction which intersects the aperture grill 11 ate fixed at the support bar 13 by a wire bracket 15 in the state where the tension is applied to both ends thereof, and the damper wire 14 offsets the vibration of the tension mask 7 due to an external sound wave generated from a speaker.
The process for manufacturing the tension mask assembly comprised of the tension mask 7 and support assembly 8 of such construction will be discussed hereinafter.
As shown in FIG. 3, the tension mask 7 is welded and fixed to the rail 12 after the rail 12 is deformed by xcex41 by the force F applied to both ends of the rail 12 of the support assembly 8. Thereafter, if the force F is removed, the rail 12 is deformed to the point where a restoring force maintains balance with a resistance of the tension mask 7, i.e., by xcex43 to an outside direction. Consequentially, the rail 12 is deformed by xcex42 as compared with an original state and this state is maintained.
The deformation amounts by the position of the rail 12 generated in this process are shown in FIGS. 4 and 5. The CRT of 76 cm (diagonal line of the screen) is taken as an example for these deformation amounts.
As shown in FIG. 5, the deformation amount (xcex42) of the rail 12 maintains balance with the force of the tension mask 7 in the state where the rail 12 is deformed by 12.7 mm from the central portion and by 2.1 mm from the end portion. That is, the central portion has greater deformation amount than the peripheral portion by 6 times and the deformation amount becomes decreased from the central portion to the peripheral portion. The reason why such a phenomenon occurs is that the peripheral portion of the rail 12 is directly welded to the support bar 13 to apply the expansion force to the rail 12, while the central portion of the rail 12 is deformed by the expansion force applied from the support bar 13 and the resistance of the tension mask 7.
FIGS. 6 and 7 show the tension and the deformation amount applied to the tension mask 7 by the expansion force of the rail 12.
FIG. 6 shows the tension by the position applied to the tension mask 7 in the state where the tension mask 7 maintains balance with the rail 12. This shows that the tension is increased from the central portion to the peripheral portion and is remarkably increased at the end of the peripheral portion where the support bar 13 is placed.
FIG. 7 shows the deformation amount of the tension mask 7 by the tension as described above. As is apparent from the figure, the tension mask 7 is deformed in proportional to the tension and the deformation amount at the peripheral portion is comparatively great.
When the deformation amount (about 0.15%) at a yield point of the material for a general tension mask 7 is set as a basis, the central portion of the tension mask 7 is below the yield point, i.e., in the elastic deformation state, and the peripheral portion exceeds the yield point and is maintained in the plastic deformation state.
The combining structure of the tension mask 7 and support assembly 8 as described above is processed by several heat treatments (about 400xcx9c450xc2x0 C.) in a high temperature furnace in the process of manufacturing the CRT. As the rail 12 and the support bar 13 are thermally expanded in this process, the expansion force is increased in a vertical direction of the rail 12 and thereby the tension applied to the tension mask 7 is increased. The peripheral portion of the tension mask 7 which is maintained in the plastic deformation state is applied with the increased tension. Thereby, the break of the aperture grill 11 or the permanent plastic deformation may occur and this causes the drooping of the peripheral grill when the peripheral portion returns to a normal temperature after the heat treatments.
If there occurs the drooping of the grill as described above, there is generated a difference between the inherent vibration frequencies by the portion due to the difference of the tension between the central portion arid the peripheral portion of the tension mask 7. Therefore, if the tension mask 7 resonates by the sound wave or shock provided from an external during the operation, the position between the aperture grill 11 and the phosphor screen 1 is comparatively displaced and thereby the striking position of the electron beam is displaced, which causes a howling, i.e., a color purity deterioration phenomenon due to the stains on the picture.
To solve these problems, Japanese Patent No. 1990-276137 proposes a method of welding and fixing a resilient support assembly 17b which has a higher rate of thermal expansion than a support bar 13b at a lower flange part of the support bar 13b as shown in FIG. 8. If the heat treatment is processed as described above, the support assembly 17b is contracted to the inner side and thus the increase of the tension applied to the tension mask 7b via the rail 12b is suppressed, thereby preventing the damage of the aperture grill 11b. 
However, the above method has a problem of increasing the number of components and the manufacturing costs, since a private resilient support assembly 17b should be placed so as to prevent the damage of the aperture grill 11b. 
That is, in the above technique, the support spring should be fixed to the support bar via another means, i.e., a spring bracket 16 when the support spring 10 for fixing the support assembly 8 to the panel stud pin 9 is fixed to the support bar 13 as described above with reference to FIGS. 1 to 2.
Moreover, the wire bracket 15 which applies the tension to the damper wire 14 also requires an additional support assembly 16xe2x80x2, thereby increasing the number of components.
It is therefore, an object of the present invention to provide a tension mask assembly of a flat CRT which can simplify the construction of an assembly for supporting a tension mask in the state where a predetermined tension is maintained and prevent the path of electron beams from breaking away by isolating the permanent plastic deformation of a grill due to a thermal expansion.
It is another object of the present invention to provide a tension mask assembly of a flat CRT which can decrease the number of components and processes by simplifying the construction of a support assembly for supporting a tension mask, down the ghost according thereto and improve the color purity of the CRT by preventing a howling due to an external vibration.
To achieve the above objects, the tension mask assembly of a flat CRT comprises a tension mask which is placed to be opposite to a phosphor screen formed at an inner surface of a panel with maintaining a predetermined distance from the phosphor screen and serves to discriminate the colors of electron beams; an aperture grill which serves as an electron beam transmitting hole and is formed on an effective surface of the tension mask to have a slot or dot shape a rail which is supported with applying a predetermined tension to both ends of a long side part or a short side part of the tension mask; a support bar which supports both ends of the rail by being arranged in a direction that crosses with the rail and has a support spring and a damper wire disposed thereon; and a tension controlling member which has a different rate of thermal expansion from the support bar and is placed in a longitudinal direction against the side wall of the support bar.
According to a first embodiment of the present invention, it is preferred that the tension controlling member is made of the material which has a higher rate of thermal expansion than the support bar.
According to a second embodiment of the present invention, it is preferred that the tension controlling member is made of the material which has a lower rate of thermal expansion than the support bar and is combined at an inside wall of the support bar.
According to a third embodiment of the present invention, it is preferred that a spring fixing part and a bracket fixing part for fixing the support spring and the damper wire are incorporated at an inside or outside of the tension controlling member.
According to a fourth embodiment of the present invention, it is preferred that the tension controlling member is placed at the longitudinal direction of the support bar with the central portion thereof maintaining a regular position with the central portion of the support bar.
If the mask assembly is heat-treated several times at the inside of a high temperature furnace in the process of manufacturing a CRT, the rail and the support bar become thermally expanded. In this case, even though the rate of expansion of the rail becomes increased, the support bar is bent to an inner direction by the tension controlling member of the present invention which has a different rate of thermal expansion and thereby the tension applied to the tension mask when the rail is contracted is not increased.
Consequentially, it is possible to prevent the grill break or permanent plastic deformation of the tension mask and the deterioration of the color purity due to the breaking away of the electron beam.
Furthermore, it is possible to improve the vibration characteristic of the tension mask and reduce the manufacturing costs by lowering the number of components and manufacturing processes.
A plurality of embodiments of the present invention can be proposed, however, a description will be made hereinafter only on the most preferable embodiment of the present invention.